One type of non-volatile memory is resistive memory. Resistive memory utilizes the resistance value of a memory element to store one or more bits of data. For example, a memory element programmed to have a high resistance value may represent a logic “1” data bit value, and a memory element programmed to have a low resistance value may represent a logic “0” data bit value. The resistance value of the memory element is switched electrically by applying a voltage pulse or a current pulse to the memory element. One type of resistive memory is phase change memory. Phase change memory uses a phase change material in the resistive memory element.
Phase change memories are based on phase change materials that exhibit at least two different states. Phase change material may be used in memory cells to store bits of data. The states of phase change material may be referred to as amorphous and crystalline states. The states may be distinguished because the amorphous state generally exhibits higher resistivity than does the crystalline state. Generally, the amorphous state involves a more disordered atomic structure, while the crystalline state involves a more ordered lattice. Some phase change materials exhibit more than one crystalline state, e.g. a face-centered cubic (FCC) state and a hexagonal closest packing (HCP) state. These two crystalline states have different resistivities and may be used to store bits of data. In the following description, the amorphous state generally refers to the state having the higher resistivity, and the crystalline state generally refers to the state having the lower resistivity.
Phase change in the phase change materials may be induced reversibly. In this way, the memory may change from the amorphous state to the crystalline state and from the crystalline state to the amorphous state, in response to temperature changes. The temperature changes to the phase change material may be achieved in a variety of ways. For example, a laser can be directed to the phase change material, current may be driven through the phase change material, or current can be fed through a resistive heater adjacent the phase change material. In any of these methods, controllable heating of the phase change material causes controllable phase change within the phase change material.
A phase change memory including a memory array having a plurality of memory cells that are made of phase change material may be programmed to store data utilizing the memory states of the phase change material. One way to read and write data in such a phase change memory device is to control a current and/or a voltage pulse that is applied to the phase change material. The level of current and/or voltage generally corresponds to the temperature induced within the phase change material in each memory cell.
The data retention performance of a phase change memory depends strongly upon the temperature history of the memory. Typically, for nonvolatile memory, data retention is guaranteed for at least ten years for operating temperatures up to 85° C. The data retention guarantee is mostly a material property and strongly depends on the crystallization temperature of the phase change material used. For example, for Ge2Sb2Te5, the real retention performance is about ten years for operating temperatures up to 95° C. to 110° C. For many applications, however, this temperature specification is not sufficient. For example, in automotive applications this temperature may be exceeded. In addition, a memory device is not typically operated at a constant ambient temperature, but rather experiences significant changes in the ambient temperature. For example, a memory device for an engine controller of a car will experience extremes in temperatures based on whether the engine is running. In this case, the data retention of the memory device is not so strongly impacted by momentary temperatures (within certain limits) or an average temperature, but rather by the temperature budget accumulated by the memory device.
For these and other reasons, there is a need for the present invention.